The invention relates to an offshore drilling unit, and more particularly to a semi-submersible vessel for conducting offshore operations in moderate or severe environments, such as the Gulf of Mexico, the North Sea, offshore Newfoundland, and the like.
In recent years, the drilling operations have been conducted at increasingly greater distance from the shoreline, placing the offshore production or drilling facilities in often severe weather conditions. In such environments it is particularly important to have a stable floating facility for supporting the mineral exploration and production operations, as well as providing living accommodations to the crew and storage for the necessary equipment. In deep waters, over 7500 feet, it becomes particularly advantageous to deploy floating semi-submersible vessels, as opposed to fixed bottom anchored structures.
Designs of semi-submersible vessels utilize buoyant pontoons, or lower hulls which support a plurality of vertically extending columns, the upper portions of which carry a working platform. Some of the semi-submersible vessels can have a single caisson, or column, usually denoted as a buoy while others utilize three or more columns extended upwardly from buoyant pontoons.
In many such structures, vertical or diagonal braces are used between the columns, the braces contributing to the water plane area of the vessel. The braces are usually constructed with smaller diameters than that of the columns and are therefore more vulnerable to the environmental and mechanical damage. If the connecting braces are damaged, the entire structure becomes jeopardized.
One example of a single-brace structure is shown in U.S. Pat. No. 4,436,050 issued on Mar. 13, 1984 to Hadar Liden entitled xe2x80x9cSemi-Submersible Vessel.xe2x80x9d The ""050 patent discloses a two-pontoon, four-column structure, with a pair of columns being mounted on a respective underwater hull, or pontoon. One transverse horizontal stay is mounted between each pair of the columns at the ends of the underwater hulls. The object of the ""050 patent design is to simplify the construction and to reduce the resistance to water flow. However, the minimal number of braces may be less beneficial where spreading forces acting on the four columns are relatively high and torque imposed on the columns by the pontoon lateral bending tends to twist a column structure in the direction of the prevailing wind and wave forces.
There also exist numerous designs of semi-submersible vessels using diagonal braces in addition to horizontal stays. These tend reinforce the support structure of the platforms and resist destructive forces of the ocean waves. Some of the designs using diagonal braces are shown in U.S. Pat. No. 4,232,625 (Goren) and U.S. Pat. No. 4,281,615 (Wilson et al.). One of the disadvantages of the diagonal braces is increase in water plane area of the vessel, which adversely affects the weight, wave resistance and overall cost of the vessel.
Another consideration that is taken into account when designing semi-submersible vessels is resistance of the vessels to heave and roll motion induced by waves. The vessels must have sufficient stability to withstand wave motions to allow the mineral exploration and production operations to be carried out in safety.
This consideration becomes particularly important in harsh environments where strong winds and waves are prevalent throughout the year. The present invention contemplates provision of a semi-submersible vessel, particularly adapted for use in hostile environment, a vessel with improved safety features and high maneuverability.
It is, therefore, an object of the present invention to provide a semi-submersible vessel with improved safety features.
It is another object of the invention to provide a semi-submersible vessel with a limited number of horizontal braces assuring minimal increase in water plane area, while resisting spreading and twisting forces acting on the supporting columns.
These and other objects of the invention are achieved through a provision of a semi-submersible vessel for use in particularly harsh environment. The vessel comprises a pair of parallel buoyant pontoons having a generally rectangular configuration and divided into a plurality of independent compartments. Four columns, or caissons are mounted on the pontoons and extend vertically to support the upper deck of an offshore platform suitable for conducting mineral exploration or production operations.
Two pairs of parallel braces are secured transversely between the pontoons, one pair of parallel horizontal braces between aft columns and one pair of parallel horizontal braces between the forward columns, the braces extending generally perpendicularly to longitudinal axes of the elongated pontoons. The vessel has a relatively small water plane area since it does not require diagonal braces to reinforce the support column structure, and the horizontal braces are relatively small in diameter.
The horizontal braces offer fixity to column twisting due to the lower hull pontoon lateral bending between the fore and aft columns caused by wave and current forces. Lateral secondary bending due to quartering seas is reduced due to the utilization of more compact bracing members having lower lateral bending inertia properties. The vessel uses no diagonal braces, thereby keeping the water plane area at a minimum.
In addition to added resistance to spreading and twisting forces acting on the columns the horizontal braces offer a certain redundancyxe2x80x94if one of the braces becomes damaged the second brace takes up the entire load of securing the pair of columns until such time as the damaged brace can be repaired.
Each horizontal brace is provided with internal xe2x80x9credundancyxe2x80x9d featuresxe2x80x94they contain two independent watertight compartments. A bulkhead divides an oblong brace into two independent sections to retain stability of the vessel even if the structural integrity of a compartment wall is compromised by wave motions or physical objects striking the brace. In that manner, even if one section of the wall is damaged, the second independent buoyant compartment will help to retain structural capability at severe environmental occurrences.
The vessel is moored at a location using dynamically positioned thruster assemblies mounted on the pontoons. The thruster unit extends below the pontoon, while a motor which independently drives a respective thruster unit, is mounted inside a thruster room within the pontoon. A passageway between the thruster rooms in the pontoons allows access to the thruster assemblies, with the passageway communicating with the platform at a level above an operating draft of the vessel.
To minimize drag forces induced by wave motions, the columns and hulls of the pontoons are provided with outwardly convex, rounded corners. This design reduces drag force transmitted when mooring or moving the vessel. The pontoon hulls have compartments for storing ballast, drill water and other necessary supplies.
The columns, or caissons house reserve mud tanks, ballast tanks, force air ventilation supply and other necessary machinery and equipment. An integrated box deck structure houses drilling and ship service equipment, power generation, storage for liquid mud and other miscellaneous objects. Living quarters and service equipment are mounted on the platform, as well.